The invention relates to an improved facility for conducting electrical power to electrodes, in particular to electrodes immersed in corrosive salt melts, amongst others for the electrolysis of fused halide salt melts, in particular a chloride melt containing dissolved aluminum chloride, and such that the said device is stable in operation and functions with a low voltage drop. This device is also suitable for equipment for heating fused salt melts with alternating currents, for electroplating articles in fused salt melts and similar equipment.
In such equipment, and in particular in the electrolysis of metals in fused halide salt melts, the design of the container for the fused salts, in particular the cell and the arrangement of the electrodes, are to a large degree determined by the fact that for corrosion reasons one must avoid passing the metallic conductor through the fused salts.
In the case of the Hall-Heroult electrolytic process for the extraction of aluminum, this is achieved by casting iron bars in the cathodic carbon floor of the cell and conducting the power into the carbon floor and the liquid metal via these bars. In the anode part the metal conductor rods are embedded and packed into the carbon blocks with pitch.
The electrolytic production of magnesium makes use of vertical electrodes. In that case graphite plates which project out of the melt are employed as the anodes, the power being passed directly into these; one accepts in that case a relatively high voltage drop, a heat loss and regular anode changes which are required because the anodes burn off at the melt/gas interface.
If now a highly conductive metal such as copper or high nickel steels are used as the conductor bars, then the voltage drop is lowered. However, at the same time the conductor bars are attacked by anode gas, liquid metal and molten salts, and, if the electrode plates are made of graphite, by carbon viz., a form of attack which no metal can withstand for very long in the temperature range 550.degree.-800.degree. C. Furthermore, the electrode material, usually graphite, if desired also a baked artificial carbon mass, always has some residual porosity. The result of this is that the melt steadily migrates through the electrode material, comes in contact with metallic conductor in the electrode and attacks it.
A solution to this problem should be of great technical advantage, that is, the conduction of electrical power to and from the electrodes of a furnace or cell containing a melt, or the anodes and cathodes of an electrolytic cell, via conductor bars which are made of highly conductive metal, and are not attacked by the ambient gases or the melt, or are effectively protected from attack by the surrounding media. This is particularly true for the electrolysis of metals from fused halide salts, for example, the production of magnesium from a chloride melt contaning dissolved magnesium chloride, alkali metals from their chlorides and finally aluminum from an alkali halide melt containing dissolved aluminum chloride.
A number of different solutions have already been proposed.
For example, the U.S. Pat. No. 3,838,384 describes a metallic conductor which passes through the cell wall and therefore is protected from corrosive attack. The conductor is provided with a protective sleeve, impervious to gases and fluids and made of graphite, which encloses the conductor after it penetrates the outer cell wall, deep into the graphite electrode. The protective sleeve is coated with highly conductive pyrolytic graphite to reduce its porosity and improve its protective characteristics. This sleeve is pressed tightly into the electrode at one end and at the other end sealed against penetration by electrolyte by means of a kind of packed box facility on the outer cell wall. In addition, this end of the conductor is cooled somewhat so that the electrolyte freezes there.
In U.S. Pat. No. 3,809,749 the same problem is solved by retaining the highly conductive graphite sleeve, which is as impervious as possible. In addition, however, a positive pressure of inert gas, higher than the pressure in the cell, is maintained between the conductor and the protective sleeve to prevent electrolyte entering the sleeve. At the same time this gas pressure or the flow of gas indicates if the electrode is damaged in any way.
Both solutions suffer from the disadvantage that the metallic conductor is protected only as long as the mechanical and physical means for preventing penetration of the corrosive surrounding media remains intact or undisturbed. An additional disadvantage is the fact that, although the metallic conductor itself exhibits a low voltage drop, the mechanical and therefore the electrical contact between the solid metal and the graphite deteriorate with time. In particular, the migration of the fused salt charge through the electrode material and finally through the graphite sleeve, even when this is coated with pyrolytic graphite, can not be prevented completely by the proposed inert gas, so that it ultimately leads to attack of the metallic conductor.
It is an object of the present invention to avoid such disadvantages and, in particular to develop a means of power conduction which is stable in operation and functions with a low voltage drop.